1. Field of the Invention
The present invention relates to an x-ray generation device and a cathode thereof. More particularly, an x-ray generation device and a cathode thereof of the present invention comprise an electron beam generator having at least one metal unit being chemical-vapor-deposited a carbon layer in the form of multiple-walls.
2. Descriptions of the Related Art
An x-ray generation device generates field emission electrons according to quantum theory of field electron emission. The basic principle of the field emission electrons is that the electrons of a conductor must have sufficient energy to get a chance to cross the potential energy barrier to the vacuum side when no electric field is applied. When an electric field is applied, the energy band is bent so electrons can cross the potential energy barrier to the vacuum side without huge amount of energy. When the applied electric field is increasing, the potential energy barrier to be crossed by electrons is decreasing and the strength of the derived current is increasing. According to electromagnetic theory, a sharp end of an object accumulates more electric charges than a blunt end of the object does. That is, a sharp end of an object has a higher electric field than an blunt end of the object does. Therefore, the electronic emitting part of a field emission cathode (i.e. x-ray generation device) is designed in the sharp form so that stronger electric field can be derived without applying high voltage.
At present time, an x-ray generation device usually serves as an electron source within a microwave element, sensor, panel display, or the like. The efficiency of electron emission mostly depends on the element structure, material, and shape of a field emission cathode (i.e. an x-ray generation device). A field emission cathode is made of metal, such as silicon, diamond, and carbon nano tube. Among these materials, carbon nano tube is particularly important because its openings are extremely thin and stable, it has low conducted field and high emitting current density, and it is highly stable. With these characteristics, carbon nano tube is extremely suitable for a field emission cathode. Therefore, it is highly possible that carbon nano tube will replace other materials and becomes the material of field emission in the next generation.
Field emission cathode can serve as a cathode of an x-ray generation device, such as an x-ray tube. An x-ray generation device encapsulates a cathode, electromagnetic-lens aperture, and an anode target within a glass container. The conventional thermionic cathode neon tube can be replaced by the carbon nano tube. When using a thermionic cathode neon tube in an x-ray generation device, around 99% of electricity is transformed to heat. Thus, the thermionic cathode neon tube must be cool down by cooling water. On the contrary, carbon nano tube can emit electron beams under smaller electric field intensity, so the efficiency of transferring electricity to electronic beams is higher than that of thermionic cathode nano tube. In addition, cooling process is not required when using carbon nano tube in an x-ray generation device.
The U.S. Pat. No. 6,553,096 presented by Zhou et al. discloses an x-ray generation device adopting carbon nano tube. Zhou et al. use materials with nanometer structures as an emitting source of a cathode field emission. Furthermore, Zhou et al. claim that 4 A/cm2 of current density can be achieved.
The technique disclosed by Zhou et al. has to firstly purify carbon nano tubes by strong acid to make carbon nano tubes being shorter than 0.5 micrometer and being in the form of single-wall. Then, the carbon nano tubes are deposited on a substrate. The advantage is that the carbon nano tubes do not have to be fixed on the substrate by adhesive. In order to generate 10 mA/cm2 of current density, 2.4 V/um to 5 V/um of starting voltage is required by the technique disclosed by Zhou et al. When a higher current density, such as 100 mA/cm2 is required, the electric field has to be increased to 4 V/um to 7 V/um.
Zhou et al. asserts that the starting voltage required by their field emission cathode (which uses carbon nano tube in cathode) is much smaller than that required by conventional field emission cathodes (which require 50 V/um to 100 V/um of starting voltage and has MO or silicon sharp end). A field emission cathode using the material of graphite powder requires 10 V/um to 20 V/um of starting voltage, which is also beaten by the technique Zhou et al. Although field emission cathode using nano diamond can lower the starting voltage to 3-5 V/um, it is unstable when the current density is above 30 mA/cm2.
Actually, the technique disclosed by Zhou et al. is very complicated. First, the graphite powder being the major material is added 0.6 atomic percent of nickel and/or 0.6 atomic percent of cobalt, and then they are placed into a quartz diode, wherein the added nickel and/or cobalt are the activator. The quartz diode is then heated up to 1150° C. The quartz diode is vacuumed and further injected with inert gases to maintain the pressure at 800 torr. Afterwards, the quartz is burned by Nd:YAG laser and then injected with inert gases again to let nano carbon be deposited on the inner wall of the quartz diode. At this time, the volume ratio of the derived signal wall nano tube is 50-70%. Thereafter, a purifying process, such as 20% H2O2, is required. The diameter of one single carbon nano tube is approximately 1.3-1.6 nm. The diameter of a bunch of carbon nano tubes is about 10-40 nm. Alternatively, the purifying process can use sulfuric acid and nitric acid with volume ration of 3:1. The length of the carbon nano tube is approximately 500 nm. In addition to the aforementioned processes, a series of deposition and lithography process to form the cathode is still required.
According to the aforementioned descriptions, an x-ray generation device and a cathode thereof having lower starting voltage is always preferred. Although carbon nano tube can achieve better performance and efficiency, the technique provided by Zhou et al is extremely complicated. Consequently, a simpler process to make an x-ray generation device and the cathode thereof is still in an urgent need.